Power semiconductor chips available today comprise a very wide spectrum of different chip technologies. In the past, the further development of rapidly switching power semiconductor chips has led to ever higher permissible barrier-layer temperatures, and consequently operating temperatures. It has consequently become necessary to replace conventional connecting techniques with which the power semiconductor chips are connected to other components of the power semiconductor module by improved connecting techniques that withstand higher temperatures. This applies all the more if the power semiconductor chips concerned are used in switching operation and, as a result, are exposed to great alternating temperature loading. However, such improved connecting techniques generally require special, expensive chip metallizations, for example with copper or precious metals. Furthermore, it may be necessary for the components of the module that are connected to such special chip metallizations, for example metallizations of ceramic substrates in which the power semiconductor chips are mounted, or bonding wires with which the power semiconductor chips are connected, also to meet increased requirements, which generally increases the costs.
The latest developments in chip metallization of power semiconductor chips are heading toward copper metallizations; the chip metallizations of conventional power semiconductor chips, on the other hand, predominantly consist of aluminum. Copper has the advantage over aluminum of an electrical conductivity that is approximately 50% higher. Furthermore, copper is particularly well-suited for the production of high-temperature-resistant diffusion soldering connections. However, copper is expensive and readily oxidizes. To produce a diffusion soldering connection, however, at least one blank copper surface is required, i.e. either the copper must be freed of an oxide film, for example, before the diffusion soldering, or else a blank copper surface must be protected from oxidation in the time period before the diffusion soldering, for example if the component concerned is to be stored for a relatively long time. However, all these measures are laborious and expensive.
Apart from diffusion soldering connections, low-temperature pressure sintering connections also have a high temperature resistance and alternating temperature resistance. Here, a paste, which contains silver powder and a solvent, is introduced between the components to be joined that are to be connected to one another. Then the components to be joined are pressed against one another under high pressure at a predetermined temperature for a predetermined time. This produces a connection that is resistant to high temperature and stable under changing temperatures. Apart from the fact that this production process as such is laborious and expensive, the surfaces of the components to be joined that are to be connected to one another must be coated with a precious metal, for example silver or gold, which likewise increases the costs.
A further conventional connecting technique is that of wire bonding. Here, a bonding wire is bonded, for example, onto an upper, freely accessible chip metallization. The bonding wires that are usually used for this purpose predominantly consist of aluminum. When operating under alternating loads with great temperature differences, however, the mechanical properties of aluminum deteriorate over time, and this is accompanied by a deterioration in the strength of the bonding connection. After being in operation for a relatively long time, this may cause the bonding wire to “lift off” from the chip metallization. As a difference from this, copper has much better properties when operated under alternating loads, for which reason copper-based bonding wires are increasingly being used. However, a high-quality wire bonding connection that is stable under changing temperatures requires that the hardness of the chip metallization and the hardness of the bonding wire do not differ too much. It is therefore advantageous from a technical viewpoint to use in the case of modern power semiconductor chips upper chip metallizations of copper, which however are expensive.
In the production of today's power semiconductor modules, a consistently applied electrical or mechanical connecting technique is used for production engineering reasons, which means that connections corresponding to one another of all the power semiconductor chips of the power semiconductor module are integrated in the module structure with the same connecting technique. The reason for this is that the same connecting technique is used for all comparable sub-steps in the production of a power semiconductor module to maintain an economic process.
For example, the underside metallizations of all the power semiconductor chips of the module may be soldered by means of a conventional fusion soldering connection, for example on metallized ceramic substrates, while the upper-side metallizations are respectively connected electrically by means of an aluminum bonding wire connection. If, however, modern, rapidly switching power semiconductor chips are fitted in the power semiconductor module with these connecting techniques, there is a high probability of failure of the power semiconductor module lying in the conventional connecting technique, since modern power semiconductor chips are usually operated over a greater temperature range than conventional power semiconductor chips.
As an alternative to the conventional connecting techniques, high-temperature-resistant connecting techniques (i.e. diffusion soldering, low-temperature pressure sintering, adhesive bonding, copper wire bonding) may of course be used consistently throughout the module at locations of the module that correspond to one another. This means, however, that the metallizations of the semiconductor chips that are not exposed to high temperatures or to great alternating temperature loading must have metallizations with copper and/or with a precious metal purely and simply to use the modern connecting technique. This means that the greater reliability of power semiconductor modules that are produced with these modern connecting techniques is bought at the price of increased production costs.